Cross-linked foamed pressure sensitive adhesive and method for preparing the same

ABSTRACT

A cross-linked foamed pressure sensitive adhesive having excellent stress relaxing property, restoration property and solvent resistance brought forth by uniform and sufficient cross-linking and foaming irrespective of transmissibility of ultraviolet rays and electron beams to components, and free from drawbacks inherent to low molecular weight acrylic copolymers. A cross-linked foamed pressure sensitive adhesive having restoration property, prepared by heating a heat cross-linkable and heat foamable composition containing a tacky polymer having a molecular weight of at least 100,000, a heat cross-linking agent and a heat foaming agent to cause cross-linking and foaming.

FIELD OF THE INVENTION

This invention relates to a cross-linked foamed pressure sensitiveadhesive and, particularly, to such an adhesive that can be usedeffectively as a buffer material, a stress relaxing material, a sealingmaterial or a combination thereof, and also to a production methodthereof.

BACKGROUND OF THE INVENTION

Since a foamed body has the property of absorbing vibration, it has beenused widely in the application as a sound-proofing material, a buffermaterial or a stress-relaxing material. As described in JapaneseUnexamined Patent Publication (Kokai) No. 9-78038 and WO99/03943, thefoamed body is sometimes used as a substrate of an adhesive tape or anadhesive sheet (hereinafter called the “adhesive tape, etc”), since thefoamed body is superior in flexibility, and is therefore easilyconformable to an adherend.

Specifically, Japanese Unexamined Patent Publication (Kokai) No. 9-78038uses, as a substrate material, a foamed body using an elastomer such asepichlorohydrin rubber or an ethylene-propylene-diene terpolymer (EPDM)for a matrix material. However, elastomers are generallydifficult-to-bond materials, and a pressure sensitive adhesive cannot beeasily applied by coating or lamination. The foamed body of theepichlorohydrin rubber, in particular, contains chlorine, and a carefulattention must be paid when this foamed body is discarded. The foamedbody of EPDM contains large quantities of process oil. This oil islikely to bleed from the EPDM foamed body and is not much desirable foruse in the pressure sensitive adhesive.

On the other hand, a foamed pressure sensitive adhesive, in which thepressure sensitive adhesive itself is a foamed body, is known, too.

For example, the WO99/03943 specification describes a foamabledouble-sided pressure sensitive adhesive tape produced by dispersing andpacking a plurality of microcapsules in a cross-linkable tacky matrixmaterial. According to the invention of WO99/03943, the microcapsulescan impart the compression restoration force to the double-sidedpressure sensitive adhesive tape, but are likely to restrict thematerials of the adhesive. As a result, the adhesive tape has a lowstress relaxing property and can rarely reduce the compressive load.

Japanese Unexamined Patent Publication (Kokai) No. 63-225684 describes afoamed pressure sensitive adhesive layer having both cross-linkedstructure and foamed structure from the aspect of improvements in heatresistance, aggregation force and stress relaxing property. Moreparticularly, according to this Japanese Unexamined Patent Publication(Kokai) No. 63-225684, an acrylic polymer having an epoxy group(hereinafter called also as the “glycidyl group”) is treated withultraviolet rays in the presence of a diazonium salt compound to formsimultaneously the cross-linked structure and the foamed structure inthe foamed pressure sensitive adhesive layer. However, the acrylicpolymer or other components must be selected carefully so as not toinhibit transmission of the ultraviolet rays. If the components make itdifficult for the ultraviolet rays to transmit because of their blackcolor etc., the degree of cross-linking and foaming of the acrylicpolymer becomes insufficient.

Electron beams may be used in place of the ultraviolet rays. However,the electron beams cannot transmit easily ordinary materials. Therefore,cross-linking by means of the electron beams may result in thelimitation of the thickness or the degree of cross-linkage.

Japanese Unexamined Patent Publication (Kokai) No. 55-90525 discloses apressure sensitive adhesive foamed body that is foamed and cross-linkedby heat-treatment in place of the UV treatment. More particularly, astarting mixture containing an acrylic type low molecular weightcopolymer having reactivity with isocyanate, polyisocyanate and afoaming agent is foamed and cross-linked by heat to give apressure-sensitive adhesive foamed body. This reference describes thatthe acrylic type low molecular weight copolymer has an average molecularweight of not greater than 10,000. When the acrylic polymer used has arelatively low molecular weight of not greater than 10,000, theaggregation force of the foam cannot be obtained sufficiently becausethe molecular chains are short. In other word, the foamed body has hightackiness on the surface and is likely to result in an aggressivepressure sensitive adhesive, and its bonding power (from normaltemperature to low temperature) is as high as that of ordinary acrylictype pressure sensitive adhesive. Therefore, this foamed body involvesthe problem that when it is cut, it adheres to a cutting blade and thecutting work becomes difficult. Since this foamed body uses the lowmolecular weight copolymer, the foam is brittle and lacks sufficienttenacity. Furthermore, since it uses the low molecular weight copolymer,the viscosity is so low that a foam having a large thickness cannot beobtained.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a cross-linkedfoamed pressure sensitive adhesive that avoids one or more of theproblems of the prior art and to provide a production method of such anadhesive. Preferably, the present invention provides a cross-linkedfoamed pressure sensitive adhesive that can be cross-linked and foameduniformly and sufficiently irrespective of transmissibility ofultraviolet rays and electron beams to its components. The presentinventive adhesive also exhibits one or more, and preferably all, of thefollowing properties: excellent stress relaxing property, excellentrestoration property, excellent solvent resistance, and freedom from theproblem resulting from a low molecular weight acrylic type copolymer.

According to one aspect of the present invention, there is provided across-linked foamed pressure sensitive adhesive having a restorationproperty, that is obtained by heating a heat cross-linkable and heatfoamable composition containing a tacky polymer having a molecularweight of at least 100,000, a heat cross-linking agent and a heatfoaming agent, to cause cross-linking and foaming.

According to another aspect of the present invention, there is provideda method for producing a cross-linked foamed pressure sensitive adhesivehaving restoration property, that comprises the step of heating a heatcross-linkable and heat foamable composition containing a tacky polymerhaving a molecular weight of at least 100,000, a heat cross-linkingagent and a heat foaming agent, to cause cross-linking and foaming.

According to the pressure sensitive adhesive and the production methodthereof, the adhesive has at least one, and preferably all of, excellentstress relaxing property, restoration property and solvent resistancebecause cross-linking and foaming are attained uniformly andsufficiently. Unlike conventional methods that conduct the UV(ultraviolet) treatment, the present invention obtains the cross-linkedfoamed pressure sensitive adhesive by conducting cross-linking andfoaming by means of heating. Therefore, foaming can be achieveduniformly and sufficiently irrespective of transmissibility of theultraviolet rays and the electron beams to the composition. Because bothfoaming and cross-linking are provided at substantially the same time tothe adhesive, the density of the resulting adhesive can be adjusted overa broad range. The resulting adhesive has high aggregation property, istenacious, and can be obtained also in the form of thick foams.

The term “cross-linked foamed pressure sensitive adhesive” used in thisspecification means a pressure sensitive adhesive which is cross-linkedand foamed by heating, and the matrix of which itself has tackiness.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be explained hereinafter with reference topreferred embodiments thereof. Needless to say, however, the presentinvention is in no way limited to these embodiments.

The heat cross-linkable and heat foamable composition for producing thecross-linked foamed pressure sensitive adhesive of the present inventioncontains a tacky polymer, a heat cross-linking agent and a heat foamingagent.

The tacky polymer is those polymers which can form the matrix of theresulting cross-linked foamed pressure sensitive adhesive and can imparttackiness to the adhesive.

The tacky polymer is generally a tacky acrylic type polymer obtained bypolymerizing a polymer precursor containing a polymerizable monomer thatcontains mainly an acrylic monomer because the acrylic type polymer iseasy to blend, has excellent weather resistance and does not exertadverse influences to the environment. This tacky acrylic type polymerhas a cross-linking group capable of forming cross-linkage upon heating.To introduce the cross-linking group, the polymer precursor describedabove contains a cross-linkable acrylic monomer having a cross-linkinggroup, in one aspect of the present invention. In other words, thecross-linking group can be introduced into the tacky polymer bypolymerizing a mixture of polymerizable monomers containing anon-cross-linkable acrylic monomer having no cross-linking group and across-linkable acrylic monomer, or a mixture of a cross-linkable acrylicmonomer with a polymerizable prepolymer obtained by prepolymerizing apolymerizable monomer containing a non-cross-linkable acrylic monomer.As another method, a cross-linking group can be introduced by effectingaddition reaction or a modification reaction of a tacky polymer obtainedby polymerizing a non-cross-linkable acrylic monomer. (In the followingdescription, the non-cross-linkable acrylic monomer will also be calledmerely the “acrylic monomer”.)

The acrylic monomer is at least one monomer selected from the groupconsisting of unsaturated mono-functional (meth)acrylate esters ofnon-tertiary alkyl alcohols having a relatively low polarity and theirmixtures. The alkyl group of the non-tertiary alcohols has about 4 toabout 12 carbon atoms. In order for the resulting adhesive to form anelastomer, the monomer described above preferably has a glass transitiontemperature (Tg) of from about −60° C. to about 200° C. as ahomopolymer. Examples of such polymerizable acrylic monomers includen-butyl acrylate, ethyl acrylate, methyl acrylate, hexyl acrylate,2-ethylhexyl acrylate, isooctyl acrylate, isononyl acrylate, dodecylacrylate, lauryl acrylate, isobonyl (meth)acrylate, methyl methacrylate,2-phenoxyethyl acrylate, benzyl acrylate and phenyl acrylate. Theseacrylate or methacrylate monomers can be used either individually or asa combination of two or more monomers.

Besides the acrylic monomers having a relatively low polarity describedabove, the polymerizable monomer may contain, whenever necessary, apolar monomer such as lower alkyl-substituted acrylamide,N-vinylpyrolidone, N-vinylcaprolactam or N,N-dimethylacrylamide, imideacrylate, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, acrylicacid, itaconic acid, fumaric acid, and maleic acid. In this case, thecontent of the polar monomer is maximum 45 parts by weight on the basisof 55 to 100 parts by weight of the low polarity acrylic monomer. Whenthe polar monomer is added to the acrylic monomer in the amount fallingwithin this range, the tackiness required for the resulting elastomercan be ensured.

As described above, the polymerizable precursor also includes thecross-linkable acrylic monomer. The cross-linkable acrylic monomer isnot particularly limited so long as it is a polymerizable monomer havinga cross-linking group, but is preferably an acrylic monomer having aglycidyl group, a hydroxyl group and a carboxyl group. When thecross-linkable acrylic monomer contains the glycidyl group, the heatresistance, the solvent resistance and the distortion resistance of thematrix of the adhesive can be improved after cross-linking.

This cross-linkable acrylic monomer is contained in the amount of 0.1 to20 parts by weight per 100 parts by weight of the non-cross-linkableacrylic monomer or the polymerizable acrylic prepolymer. When the amountof the cross-linkable acrylic monomer is less than 0.1 parts by weight,sufficient cross-linkage cannot be obtained with the remarkable drop ofthe heat resistance, the solvent resistance and the distortionresistance of the foamed body. When the amount of the cross-linkableacrylic monomer exceeds 20 parts by weight, on the contrary, the glasstransition temperature becomes so high that tackiness drops remarkably.

The tacky polymer can be obtained by polymerizing the polymerizableprecursor by radiation polymerization that uses the ultraviolet rays orthe electron beams (EB). The polymerization of the polymerizableprecursor can be carried out by any polymerization method such assolution polymerization, emulsion polymerization, suspensionpolymerization and bulk polymerization in the presence of apredetermined initiator. Preferred among them is bulk polymerizationbecause it eliminates disposal of the solvent, etc, its process is easyto carry out, it has high productivity, and it does not spoil theenvironment.

When the UV polymerization is conducted by any of the means describedabove, the initiator required for the polymerization is not limited, inparticular. Examples are benzoine alkyl ether, benzophenone, benzylmethyl ketal, hydroxycyclohexylphenyl ketone, 1,1-dichloroacetophenone,2-chlorothioxanthone, and so on. It is possible to use, for example,radical polymerization initiators commercially available with thefollowing trademarks, such as “Irgacure” of Chiba Specialty Chemicals,“Dalocure” of Merck Japan, “Velsicure” of Bellsicol Co. Such aphoto-polymerization initiator is generally used in the amount of about0.01 to about 5 parts by weight per 100 parts by weight of the acrylatemonomer.

When the polymerizable precursor contains the polymerizable prepolymer,the polymerizable prepolymer is first synthesized from an acrylicmonomer other than a crosslinkable acrylic monomer by prepolymerization.The prepolymerization can be carried out by any polymerization methoddescribed above in the presence of the initiator described above.

A chain transfer agent is preferably added to the polymerizableprecursor with a predetermined amount of the initiator. The chaintransfer agent controls a polymerization. A molecular weight of thetacky polymer obtained by polymerization of the polymerizable precursoris at least 100,000, generally 100,000 to 5,000,00 and preferably100,000 to 2,000,000. When the molecular weight is within this range,the foam density, the surface tackiness and appearance of the adhesiveafter cross-linking and foaming can be adjusted excellently by selectingthe blending amounts of suitable heat cross-linking agent and heatfoaming agent. Here, although polymerization of the polymerizableprecursor can be completed in the above polymerization procedure, it isalso possible to polymerize the polymerizable precursor at 50-100% andto complete polymerization in order to obtain the tacky polymer havingthe above molecular weight range during the formulation with a heatcross-linking agent and a heat foaming agent.

Concrete examples of the chain transfer agent are halogenatedhydrocarbons such as carbon bromide and sulfur compounds such asisooctyl thioglycolate, octyl thioglycolate, lauryl mercaptan and butylmercaptan.

According to the present invention, the heat cross-linkable and heatfoamable composition further contains a heat foaming agent and a heatcross-linking agent in addition to the tacky polymer described above.The heat foaming agent is decomposed by heat and generates gases such ascarbon dioxide, nitrogen or ammonia, and imparts a foamed structure tothe resulting adhesive. Examples of the heat foaming agent are (i)inorganic foaming agents such as ammonium hydrogencarbonate and ammoniumnitrite, (ii) nitroso compounds such as N,N′-dinitrosopentamethylenetetramine (DPT), (iii) azo compounds such as azodicarbonamide (AZC) andazo bis-isobutylonitrile (ABIN), and (iv) sulfonyl hydrazide compoundssuch as benzenesulfonyl hydrazide (BSH), toluenesulfonyl hydrazide (TSH)and p,p′-oxybis(benzenesulfonyl hydrazide) (OBSH). Known cofoamingagents may be used in combination, whenever necessary, to improve thefoaming rate and to lower the foaming temperature.

The heat cross-linking agent has sensitivity to heat and can formcross-linking bonds between the tacky polymers. The cross-linking bondcan impart the heat resistance, the solvent resistance and thedistortion resistance to the resulting adhesive. Examples of the heatcross-linking agent are dithiocarbmates (such as zincdimethyldithiocarbamate, zinc diethyldithiocarbamate, zincdi-n-butyldithiocarbanmate, iron dimethyldithiocarbamate, sodiumdimethyldithiocarbamate, etc), ammonium organic carboxylates, variouspolyamines and imidazole/acid anhydrides.

In this way, the heat foaming agent and heat cross-linking agent canform the foamed structure and the cross-linking structure in thecross-linked foamed pressure sensitive adhesive. The cross-linkable andheat foamable composition for the cross-linked foamed pressure sensitiveadhesive according to the present invention does not always have totransmit radiation such as the ultraviolet rays and the electron beamsin order to furnish the cross-linked foamed adhesive with the foamedstructure and the cross-linked structure. Therefore, when thecomposition of the present invention contains the radiation-impermeablecomponents, it is more advantageous than the conventional UVcross-linkable/foamable composition because it can generate uniform andsufficient cross-linking and foaming. Examples of theradiation-impermeable components are pigments or colorants such as ablack pigment, and metal fillers or inorganic fillers for forming ashield layer such as lead powder, iron powder, titanium oxide, zincoxide, iron oxide, cerium oxide, and so forth. Since radiationtransmissibility is not affected, the adhesive is not substantiallylimited from the aspect of the size and shape inclusive of thethickness. As a matter of fact, the thickness of the cross-linked foamedadhesive according to the present invention is generally 0.1 mm to 20cm, suitably 0.2 mm to 10 cm, and most suitably 0.5 to 5 cm.

In the present invention, the heat foaming agent and the heatcross-linking agent should be selected suitably so that cross-linkingand foaming occur substantially simultaneously by heating, or foamingcan occur somewhat earlier than cross-linking. More concretely, it ispossible to select the combination of iron dimethylthiocarbamate and4,4-oxybis(benzenesulfonylhydrazide). The density and the size of thecells can also be controlled, when the blending amounts of the heatfoaming agent and the heat cross-linking agent, and the molecular weightof the tacky polymer, are adjusted suitably. In practice, when saidblending amounts the heat foaming agent and the heat cross-linking agentand the molecular weight of the polymer are adjusted in this way, thedensity of the cross-linked foamed adhesive, when it does not containthe additive such as a filler, can be controlled generally to 0.02 to8.0 g/cm³, suitably to 0.05 to 5.0 g/cm³ and more suitably 0.1 to 3.0g/cm³, and the size of the cells can be controlled within the range of10 μm to 1 nm. When the molecular weight of the polymer is 100,000 to5,000,000, the blend amounts of the heat foaming agent and the heatcross-linking agent required for controlling the density and the cellsize to the range described above are 0.01 to 10 parts by weight for theheat foaming agent per 100 parts by weight of the polymer and 0.01 to 20parts by weight for the heat cross-linking agent per 100 parts by weightof the polymer.

When the foaming temperature of the heat foaming agent is lower than thereaction start temperature of the heat cross-linking agent, cells areformed before cross-linking starts occurring. Therefore, the cells canbe disposed advantageously on the surface of the cross-linked foamedadhesive, due to diffusion. When the adhesive is bonded to an adherend,air is generally incorporated between the adhesive and the adherend.Unless this air is escaped by any means, the bonding area cannot besecured sufficiently due to the air existing between the adherend andthe adhesive. Consequently, the bonding power is likely to becomeinsufficient, and smooth bonding cannot be made. When the cells aredisposed on the surface of the adhesive as described above, however, thecells on the surface constitute channels, so that the air between thecross-linked foamed adhesive and the adherend can be discharged easilyto outside. As a result, the adhesive and adherend have a 100% contactsurface, the initial bonding power is sufficiently exhibited, andappearance after bonding is extremely excellent.

According to the present invention, a pressure sensitive adhesive can beobtained with a low density and a low compressive load in thecompressive load test as described below, as well as a sufficientrestoration property. Here, the term “restoration property” means theability of the adhesive to recover to its original form after itsdeformation. On the other hand, the term “compressive load in thecompressive load test” is an index of how easily the adhesive can bedeformed when compressed. In other words, if the compressive load of theadhesive is low, the adhesive is soft and can be deformed by applicationof a low pressure. A restoration property can be imparted to a pressuresensitive adhesive by charging heat expansible microspheres consistingof micro-capsules into the adhesive, and heating and expanding the heatexpansible microspheres. The density of this type of adhesive can bereduced by heat expansion of the microspheres. However, if themicrospheres are added to the adhesive in order to reduce the density ofthe adhesive to as low as that of the crosslinked foamed pressuresensitive adhesive according to the present invention, the compressiveload cannot be reduced, since the microspheres themselves have rigidityand thus prohibit lowering of the compressive load. On the other hand, acrosslinked foamed pressure sensitive adhesive according to the presentinvention does not have to include microspheres in order to reduce thedensity of the adhesive. Since the density of this foamed pressuresensitive adhesive can be reduced only by means of a gas, thecompressive load and density of the adhesive can be lowered and thus theresulting adhesive has excellent stress-relaxing properties.Specifically, the compressive load of the adhesive according to thepresent invention, when the adhesive having an initial thickness of 10mm is compressed to 25% of its initial thickness at a rate of 10 mm/mmin the thickness-wise direction, can be lowered from 50 N/cm² to 0.1N/cm², in the case where the density of the adhesive is lowered from 3.0g/cm³ to 0.1 g/cm³.

The cross-linked foamed adhesive described above can be shaped into thesheet form by using a heat cross-linkable and heat foamable compositionin the following way.

The tacky polymer, the heat cross-linking agent and the heat foamingagent, that are prepared in advance, are kneaded by using a uniaxial orbiaxial extruder, a Banbury mixer, a kneader, or an intermix, to give aheat cross-linkable and heat foamable composition. Next, thiscomposition is shaped into a sheet form by rolling using a heat pressmachine or a calendar roll, or by extrusion using a die, at atemperature lower than the activation temperatures of both heatcross-linking agent and heat foaming agent such as 60 to 100° C. Thissheet is then passed through an oven or a funnel cure, and is heated toa temperature higher than the activation temperatures of both heatcross-linking agent and heat foaming agent, such as 140 to 180° C., tocause foaming and cross-linking.

Though the present invention has been explained about its embodiment,but is not limited thereto.

For example, it is possible to add suitably a non-cross-linkable plasticmatrix to the heat cross-linkable and heat foamable composition,whenever necessary, so as to impart mechanical strength, elasticity andcold impact resistance to the cross-linked foamed adhesive, such asrigidity, tensile strength and elongation. Examples of such anon-cross-linkable thermoplastic matrix are various rubbers, elastomerssuch as polyethylene or polypropylene resin, styrene-butadiene rubber(SBR), acrylonitrile-butadiene rubber (NBR), polybutadiene rubber (BR),butyl rubber (IIR), styrene-isoprene-styrene block copolymer (SIS),styrene-butadiene-styrene block copolymer (SBS), andstyrene-ethylene/butylene-styrene block copolymer (SEBS), andthermoplastic polymers.

Besides the mere addition described above, it is also possible to add across-linkable plastic matrix to the heat cross-linkable and heatfoamable composition so as to incorporate it into a part of thecross-linked structure of the cross-linked foamed composition. Examplesof the cross-linkable plastic matrix are granular elastomers orelastomers prepared by adding an epoxy group or a hydroxyl group, acarboxyl group, a chlorine group or an active chlorine group to theolefin resin, the synthetic rubber or the cross-linked rubber describedabove, or a cyanate esters or poly(ethyl oxazoline) having a structureanalogous to the epoxy group. In such a case, the compressive permanentset resistance and toughness can be improved because the cross-linkagedevelops. Particularly, the elastomer can improve the cold resistance ofthe cross-linked foamed adhesive. The isocyanate ester or poly(ethyloxazoline) can react with the epoxy group (glycidyl group) in theabsence of a catalyst and can improve the heat resistance.

It is further possible to add heat expansible micro-spheres to thecross-linkable and foamable composition so as to adjust suitably theproperties of the cross-linked foamed adhesive (such as compressivepermanent set and repulsive power). Particularly when the heatexpansible micro-spheres have hollow portions incorporating low boilingpoint hydrocarbons, the density can be lowered in proportion to theaddition amount. Since the shell itself of the micro-spheres does nothave tackiness, the punching property of the matrix of the adhesive canbe improved by adding the micro-spheres. Since the micro-spheresrestrict the matrix of the adhesive, the strength in the shearingdirection can be improved. However, caution should be taken such thatthe stress relaxing property of the adhesive does not drop excessively.

Organic and inorganic fillers may be further added so as to improvedynamic performance and processing property of the cross-linked foamedadhesive and to lower the product cost. Examples of the inorganicfillers are metal oxides such as carbon black, silicic acid, silicates,carbonates, titanium oxide and zinc oxide, metal fibers and glassbubbles. Examples of the organic filler are high styrene resin,cumarone-indene resin, phenol resin, lignin or powdery rubber, andplastic bubbles. If the fillers including heat expansible microspheresare formulated into the pressure sensitive adhesive of the presentinvention, a compressive load of the adhesive can be increased to 300N/cm², when an initial thickness of the adhesive is 10 mm, and theadhesive is compressed to 25% of an initial thickness at a rate of 10mm/min in a thickness-wise direction.

Various additives such as an anti heat-aging agent, an antiozone-degradation agent, a softening agent, a plasticizer, a thickener,a lubricant, a colorant, an antistatic agent, an antimicrobial agent, aUV absorber, a flame retardant, and so forth, may be further added,whenever necessary.

The cross-linked foamed pressure sensitive adhesive according to thepresent invention may have a multi-layered structure comprising two ormore pressure sensitive adhesives depending on the kind of adherend, andmay contain a film, a non-woven fabric and a woven fabric. They can beproduced by means such as multi-layered co-extrusion or lamination.

The present invention uses a tacky polymer having a relatively highmolecular weight of at least 100,000. Therefore, the pressure sensitiveadhesive of the present invention has the following advantages incomparison with the conventional heating type pressure sensitiveadhesives. (1) Since the foam itself has low tackiness, it can be easilypeeled from the adherend after it is bonded, and has thereforere-peelability. Accordingly, the adhesive of the present invention isextremely advantageous for bonding to an adherend having a large area.(2) Since the viscosity of the composition before foaming is relativelyhigh and can be coated with a large thickness, a thick foam can beformed. (3) Cutting process is easy. (4) A tenacious adhesive can beobtained.

The resulting adhesive can be used as a sealing material for buryingdiscontinuous portions such as gaps, or a sound-proofing material.Particularly, the adhesive can be used as a flexible adhesive forburying the gaps with a substrate when an interior trim of an automobileis disposed at predetermined positions. Since the adhesive of thepresent invention has re-peelability, it can be fitted to the adherendor the substrate, then removed, and thereafter fitted once again.

EXAMPLES

1. Preparation of Samples

Samples of sheet materials were produced in the following way.

Example 1

First, polymerizable monomers and an initiator were charged into a jarto prepare a mixture. In this example, 80 parts by weight of2-ethylhexyl acrylate and 20 parts by weight of N,N-dimethylacrylamidewere used as the polymerizable monomers. 0.04 parts by weight of aphoto-initiator, that was commercially available under the trade name“Irgacure 651” from Chiba Specialty Chemicals, was used as theinitiator. Purging from the jar was conducted by using nitrogen.Ultraviolet rays were irradiated to the mixture from a fluorescent blacklamp (Sylvania F20T12B) in which 90% of the radiation rays were 300 to400 nm and which had maximum at 351 nm. This photo-initiator wasactivated to start to polymerize the polymerizable monomers and toprepare the polymerizable prepolymer. In this example, thispolymerization was continued until the viscosity of the prepolymerreached about 3,000 mPa·S (25° C.).

Next, while the mixture containing the prepolymner described above wasbeing stirred, 3 parts by weight of a cross-linkable acrylic monomerconsisting of glycidyl methacrylate, 0.1 parts by weight of thephoto-initiator consisting of Irgacure (trade name) and 0.03 parts byweight of a chain transfer agent consisting of carbon tetrabromide wereadded. After the jar was degassed, the ultraviolet rays described abovewere again irradiated to the mixture to further polymerize the unreactedmonomers in the mixture and to prepare a tacky polymer. When measured byGPC using HP1090 series II of Agilent Co, the resultant polymer wasfound to have a molecular weight of at least 100,000, and 80% of thetotal molecular weight distribution was occupied by polymers having amolecular weight of 100,000 to 5,000,000, and 75% of the total molecularweight distribution was occupied by polymers having a molecular weightof 100,000 to 2,000,000. Thus, this polymer has a molecular weight of atleast 100,000.

Next, the tacky polymer was charged into a biaxial extruder and waskneaded at 80° C. Thereafter, 1.0 parts by weight of stearic acid as alubricant, 30 parts by weight of SRF carbon black as a filler, 1.5 partsby weight of zinc dimethyldithiocarbamate and 1.0 parts by weight ofiron dimethyldithiocarbamate as heat cross-linking agents, and 5.0 partsby weight of 4,4-oxybis(benzenesulfonyl hydrazide) as a heat foamingagent were added from the intermediate portion of the cylinder of thebiaxial extruder to form a heat cross-linkable and heat foamablecomposition. This heat cross-linkable and heat foamable composition wasextrusion-molded through an extrusion die to obtain a sheet having athickness of 1 mm.

Next, this sheet was put into an oven and heat-treated at 170° C. for 15minutes to complete foaming and cross-linking and to obtain a sample ofa cross-linked foamed adhesive. The adhesive had a thickness of 4 mm.

Example 2

A sample of a cross-linked foamed adhesive was produced in the same wayas in Example 1 with the exception that a monomer component consistingof 85 parts by weight of 2-ethylhexyl acrylate and 15 parts by weight ofN,N-dimethylacrylamide was used in place of 80 parts by weight of2-ethylhexyl acrylate and 20 parts by weight of N,N-dimethylacrylamide.When measured by GPC using HP1090 series II of Agilent Co, the resultantpolymer was found to have a molecular weight of at least 100,000, and80% of the total molecular weight distribution was occupied by polymershaving a molecular weight of 100,000 to 5,000,000, and 75% of the totalmolecular weight distribution was occupied by polymers having amolecular weight of 100,000 to 2,000,000. Thus, this polymer has amolecular weight of at least 100,000. The adhesive had a thickness of 5mm.

Example 3

A sample of a cross-linked foamed adhesive was produced in the same wayas in Example 1 with the exception that a monomer component consistingof 89 parts by weight of 2-ethylhexyl acrylate and 11 parts by weight ofN,N-dimethylacrylamide was used in place of 80 parts by weight of2-ethylhexyl acrylate and 20 parts by weight of N,N-dimethylacrylamide.When measured by GPC using H1090 series II of Agilent Co, the resultantpolymer was found to have a molecular weight of at least 100,000, and80% of the total molecular weight distribution was occupied by polymershaving a molecular weight of 100,000 to 5,000,000, and 75% of the totalmolecular weight of distribution was occupied by polymers having amolecular weight of 100,000 to 2,000,000. Thus, this polymer has amolecular weight of at least 100,000. The adhesive had a thickness of 3mm.

Example 4

A sample of a cross-linked foamed adhesive was produced in the same wayas in Example 1 with the exception that 30 parts by weight of an epoxytype acryl rubber having an epoxy group as a cross-linkable group (NopolAR53L, a product of Nippon Zeon K.K.) was further added from a feedportion at an intermediate part of the cylinder of the biaxial extruder.When measured by GPC using HP1090 series II of Agilent Co, the resultingpolymer was found to have a molecular weight of at least 100,000, and80% of the total molecular weight distribution was occupied by polymershaving a molecular weight of 100,000 to 5,000,000, and 75% of the totalmolecular weight distribution was occupied by polymers having amolecular weight of 100,000 to 2,000,000. Thus, this polymer has amolecular weight of at least 100,000. The adhesive had a thickness of2.8 mm.

Example 5

A sample of a cross-linked foamed adhesive was produced in the same wayas in Example 1 with the exception that 80 parts by weight of butylacrylate was used in place of 80 parts by weight of 2-ethylhexylacrylate. When measured by GPC using HP1090 series II of Agilent Co, theresultant polymer was found to have a molecular weight of at least100,000, and 80% of the total molecular weight distribution was occupiedby polymers having a molecular weight of 100,000 to 5,000,000, and 75%of the total molecular weight distribution was occupied by polymershaving a molecular weight of 100,000 to 2,000,000. Thus, this polymerhas a molecular weight of at least 100,000. The adhesive had a thicknessof 5 mm.

Example 6

A sample of a cross-linked foamed adhesive was produced in the same wayas in Example 1 with the exception that extrusion molding was conductedto obtain a sheet having a thickness of 10 mm in place of 1 mm inExample 1. When measured by GPC using HP1090 series II of Agilent Co,the resultant polymer was found to have a molecular weight of at least100,000, and 80% of the total molecular weight distribution was occupiedby polymers having a molecular weight of 100,000 to 5,000,000, and 75%of the total molecular weight distribution was occupied by polymershaving a molecular weight of 100,000 to 2,000,000. Thus, this polymerhas a molecular weight of at least 100,000. The adhesive had a thicknessof 40 mm.

Comparative Example 1

Monomers consisting of 90 parts by weight of isooctyl acrylate and 10parts by weight of acrylic acid, 0.14 parts by weight of Irgacure 651(trade name) and 0.03 parts by weight of a chain transfer agent, i.e.2-ethylhexyl thioglycolate (OTG, product of Wako Junyaku K. K.), weremixed inside a jar. Ultraviolet rays were irradiated to this mixture topolymerize the monomer and to prepare a polymer.

The mixture was charged into a biaxial extruder with a heat foamingagent consisting of 5.0 parts by weight of4,4-oxybis(benzenesulfonylhydrazide) and was mixed. The mixture was thenextrusion-molded through an extrusion die to obtain a sheet having athickness of 1 mm. The sheet was heat-treated at 170° C. for 7 minutes,and only foaming was completed. Subsequently, electron beams acceleratedby a voltage of 300 KeV were irradiated once in a dose of 16 Mrad toboth surfaces of this sheet to achieve cross-linking and to produce acomparative sample. The adhesive had a thickness of 5 mm.

Comparative Example 2

A polymer was prepared in the same way as in Comparative Example 1.Unlike Comparative Example 1, however, the polymer of this comparativeexample was heated to 80° C. after being charged into the biaxialextruder and was kneaded. Also, 4.0 parts by weight of microcapsules(F-80D, trade name, a product of Matsumoto Yushi-Seiyaku K. K.) wasfurther supplied from an intermediate part of the cylinder of thebiaxial extruder and was mixed with the polymer. Thereafter, the polymerwas passed through an extrusion die that was heated in advance to 180°C. While the polymer was caused to foam by the microcapsules, it wasextrusion-molded into a sheet having a thickness of 1 mm. Subsequently,electron beams accelerated by a voltage of 300 KeV were irradiated oncein a dose of 6 Mrad to both surfaces of the sheet to achievecross-linking and to prepare a comparative sample. The adhesive had athickness of 1.0 mm.

Comparative Example 3

Monomers consisting of 90 parts by weight of isooctyl acrylate and 10parts by weight of acrylic acid and 0.14 parts by weight of Irgacure 651(trade name) were mixed inside a jar. The ultraviolet rays describedabove were then irradiated to this mixture and activated the initiatorso that the monomer could be polymerized to prepare a prepolymer. Thispolymerization was continued until the viscosity of the prepolymerreached about 3,000 mPa·s (25° C.).

Next, while the mixture containing the prepolymer was being mixed, across-linkable acrylic monomer comprising 0.1 part by weight of Irgacure651 (trade name) and a cross-linkable monomer consisting of 0.8 parts byweight of 1,6-hexanediol diacrylate (HDDA), 6 parts by weight of hollowglass micro-spheres (glass bubbles C15-250, product of 3M Co), 1.5 partsby weight of a filler consisting of hydrophobic silica (R-972, productof Nippon Aerosol K. K.) and 3.0 parts by weight of a surfactant wereadded to the mixture. While the mixture was being transferred to abubbler rotating at 900 rpm, and the foamed mixture was passed through apipe having a diameter of 12.5 mm and was delivered between the nips ofroller coaters at which a pair of transparent polyethylene terephthalatefilms oriented in biaxial directions and having low bondability on thesurface were disposed. Polymerization and cross-linking were completedby the irradiation of ultraviolet rays 90% of which was within thewavelength band of 300 to 400 nm and which had maximum at 351 nm. Inthis way, a comparative sample having a thickness of 1.0 mm wasproduced.

Comparative Example 4

A comparative sample of a foamed sheet (Eptosealer No. 685, product ofNitto Denko K. K.: 5.0 mm) that consists of EPDM and is used generallyin the sealing application was provided.

2. Evaluation of Samples

The sample of each Example and Comparative Example was evaluated by thefollowing measurement and test.

(1) Compressive Load Measurement

Each sheet sample was cut to obtain several square sheets of 25 mm×25mm. The sheets were laminated with one another in such a fashion as todischarge the air bubbles between them, and a test piece having athickness of about 10 mm was produced (For example 6, a sample having athickness of 40 mm was used as a test piece.). After the correctthickness (hereinafter called the “initial thickness”) was measured foreach test piece, each test piece was compressed at a compression rate of10 mm/min by using a compression tester (AUTOGRAPH, product of ShimazuSeisakusho K. K.). The compressive loads at the points at which thethickness of the test piece reached 25% and 40% of the initial thicknesswere determined.

(2) Measurement of Compressive Permanent Strain

After the correct thickness (t₀) of the test piece described above wasmeasured, the test piece was compressed at a compression rate of 10mm/min by using the compression tester described above to a thickness of40% of the initial thickness. The test piece so compressed was leftstanding with this thickness under the standard state (temperature 23°C.±1° C., relative humidity 50±2%) for 24 hours. The test piece was thenremoved from the compression tester and the final thickness (t₁) aftercompression was measured, The compression permanent strain C (%) wascalculated from these initial thickness (t₀) and final thickness (t₁) inaccordance with the following equation:C=1−(t ₁ /t ₀)×100(3) Restoration Test

After the correct initial thickness (t₀) of the test piece describedabove was measured, the test piece was compressed at a compression rateof 10 mm/min by the compression tester also described above to athickness of 80% of the initial thickness. The test piece so compressedwas left standing under the standard state. The test piece was thenremoved from the compression tester. The final thickness aftercompression was measured to check whether or not the initial thicknesscoincided with the final thickness. The test pieces were deemed to passwhen the ratio (initial thickness/final thickness) was 1.1 or below, andwere deemed to fail when the ratio 1.2 or more.

(4) Surface Tack Test

The test piece described above was pressed to a white coated panel byusing a 2 kg roller. This white coated panel was obtained by applying anacryl-melamine paint currently used as a car paint to a stainless steelpanel, and then causing cross-linking. The white coated panel was thenerected in a vertical direction, and whether or not the panel body fallsby its own weight was inspected. The test piece was deemed to pass whenthe panel body did not fall, and was deemed to fail if it did.

(5) Wet Surface Area Test

The test piece was cut into 50 mm×50 mm and was merely bonded to one ofthe surfaces of a transparent acryl sheet having a thickness of 5 mmwithout using a spatula, or the like. The bonding area (S mm²) of thetest piece to the acryl sheet was measured. A wet area ratio (W) wascalculated by the following equation:W(%)=S/2,500 mm² ×100(6) Solvent Resistance Test

The test piece was cut into a rectangle of 10 mm×20 mm, was immersed ina solvent consisting of methyl ethyl ketone (MEK) and was left standingfor 24 hours to check whether or not it is swollen. The test piece wasdeemed to pass when it did not swell by visual inspection and was deemedto fail if it did.

Table 1 illustrates the results of the measurement/tests described abovefor Examples and Comparative Examples.

It could be seen from Table 1 that the samples of Examples 1 to 6 werecross-linked sufficiently, did not generate the compressive strain, buthad the restoration property and the solvent resistance. In contrast,the sample of Comparative Example 1 had a relatively low density, hencea low compressive load, but was inferior in the restoration property asproved by leaving the compressive strain, and did not have the solventresistance. This was presumably because cross-linkage was notsufficient. The sample of Comparative Example 2 had the restorationproperty but obviously had a relatively high compressive load incomparison with its low density. It did not have the solvent resistance.The sample of Comparative Example 3 was inferior in the restorationproperty as proved by leaving the compressive strain, and did not havethe solvent resistance. It could be understood that this sample couldnot reduce either the density or the compressive load due to theproduction process. This was because this comparative example did notuse the foaming agent and cells were formed mechanically by using thebubbler. It could been seen that the sample of Comparative Example 4 notonly had the solvent resistance but also could decrease the density andthe compressive load. However, this sample did not have the surface tackby itself, and an adhesive had to be interposed between the sample andthe adherend to bond them together.

In connection with the wet surface area test, Examples 1 to 6 exhibitedthe excellent wet area ratios in comparison with Comparative Examples 2,3 and 4. Since Examples 1 to 6 could easily discharge air between theadhesive and the adherend, they had a 100% contact surface, exhibitedsufficiently the initial bonding strength and provided extremely goodappearance after bonding.

Unlike the conventional pressure sensitive adhesive that are subjectedto the UV treatment, the cross-linked foamed pressure sensitive adhesiveaccording to the present invention is one obtained by effectingcross-linking and foaming by means of heat. Therefore, cross-linking andfoaming are achieved sufficiently and uniformly irrespective oftransmissibility of the ultraviolet rays and the electron beams to thecomponents of the composition. Since cross-linking and foaming areallowed to occur uniformly and sufficiently, the adhesive of the presentinvention has excellent stress relaxing property, restoration propertyand solvent resistance. Since foaming and cross-linking are providedsimultaneously by heat to the adhesive, the density of the resultingadhesive can be adjusted over a broad range. In comparison with theconventional heat cross-linking type adhesive, the foam itself of theadhesive of the present invention has weak bonding power. Therefore,when the foam is bonded to the adherend, the adhesive can be easilypeeled from the adherend, or in other words, can have re-peelability.Therefore, the adhesive of the present invention is extremelyadvantageous for bonding to adherend having a large area. Thecomposition has a relatively high viscosity before foaming and can becoated with a large thickness. Therefore, a thick foam can be formed.Furthermore, cutting process is easy in the adhesive of the presentinvention, and a tenacious adhesive can be obtained.

TABLE 1 Example Example Example Example Example Example 1 2 3 4 5 6density g/cc 0.24 0.2 0.34 0.35 0.2 0.28 compressive N 7.6 2.7 11.3 15.84.6 5.5 load (25%) compressive N 12.7 4 18.8 26.3 7.8 12.1 load (40%)compressive % 0 0 0 0 0 0 strain restoration pass pass pass pass passpass property surface tack pass pass pass pass pass pass test wet area %100 100 100 100 100 100 solvent pass pass pass pass pass pass resistancetest Comparative Comparative Comparative Comparative Comparative Example1 Example 2 Example 3 Example 4 Example 5 density g/cc 0.2 0.25 0.520.15 0.27 compressive N 9.6 72.7 37.8 2.4 10.4 load (25%) compressive N20.6 113.4 74.5 3.9 17.4 load (40%) compressive % 2 0 3.8 0 0 strainrestoration failure pass failure pass pass property surface tack passpass pass failure failure test wet area % 100 75 85 not bonded * * *solvent failure failure failure pass pass resistance test Note) ***:Test was not conducted.

1. A method for producing a pressure sensitive adhesive comprising: (a)rolling into a sheet or extruding a mixture comprising a tacky acrylicpolymer having a weight average molecular weight of at least 100,000g/mol, a heat cross-linking agent, and a heat foaming agent thatdecomposes to generate a gas upon heating, wherein the mixture is freeof expandable, polymeric microspheres, and (b) heating the mixture to atemperature higher than the activation temperatures of both thecross-linking agent and the heat foaming agent to cause foaming andcross-linking at substantially the same time, wherein the resultingcross-linked foamed pressure sensitive adhesive has a restorativeproperty.
 2. The method of claim 1, wherein when said pressure sensitiveadhesive has an initial thickness of 10 mm and is compressed to 25 % ofthe initial thickness at a rate of 10 min/mm in a thickness-wisedirection, a compressive load is 0.1 to 300 N/cm².
 3. The method ofclaim 1, wherein the tacky polymer is obtained by polymerizing apolymerizable precursor comprising a cross-linikable acrylic monomer. 4.The method of claim 1, wherein the mixture further comprises aradiation-impermeable component.
 5. The method of clam 1, wherein thepressure sensitive adhesive has a density of 0.1 to 3.0 g/cm³ .
 6. Amethod for producing a pressure sensitive adhesive comprising: (a)rolling into a sheet or extruding a mixture comprising a tacky acrylicpolymer having a weight average molecular weight of at least 100,000g/mol, a heat cross-linking agent, and a heat foaming agent thatdecomposes to generate a gas upon heating, wherein the foamingtemperature of the heat foaming agent is lower than the reaction starttemperature of the heat cross-linking agent, and wherein the mixture isfree of expandable, polymeric microspheres, and (b) heating the extrudedmixture to a temperature higher than the activation temperatures of boththe cross-linking agent and the heat foaming agent to cause foaming andcross-linking, wherein the resulting cross-linked foamed pressuresensitive adhesive has cells that are exposed on a surface of thepressure sensitive adhesive.
 7. The method of claim 1, wherein the tackypolymer is obtained by polymerizing a non-cross-likable acrylic monomerand a cross-likable acrylic monomer.
 8. The method of claim 3, whereinthe cross-likable acrylic monomer is present in an amount of 0.1 to 20parts by weight per 100 parts by weight of the polymerizable prepolymer.9. The method of claim 7, wherein the cross-likable acrylic monomer ispresent in an amount of 0.1 to 20 parts by weight per 100 parts byweight of the non-cross-likable acrylic monomer.
 10. The method of claim6, wherein said pressure sensitive adhesive has a thickness of up to 10cm.
 11. The method of claim 6, wherein said rolling or extruding stepcomprises rolling or extruding at a temperature lower than foamingtemperature and the reaction start temperature of the heat cross-linkingagent.
 12. The method of claim 6, wherein said rolling or extruding stepcomprises an extruding step.
 13. The method of claim 6, wherein saidmixture further comprises a non-cross-linkable rubber, elastomer, orthermoplastic polymer.
 14. A method for producing a pressure sensitiveadhesive comprising: (a) rolling into a sheet or extruding a mixturecomprising a tacky acrylic polymer having a weight average molecularweight of at least 100,000 g/mol, a heat cross-linking agent, and a heatfoaming agent that decomposes to generate a gas upon heating, whereinthe foaming temperature of the heat foaming agent is lower than thereaction start temperature of the heat cross-linking agent, and whereinthe mixture is free of expandable, polymeric microspheres and saidrolling or extruding step is performed at a temperature lower than (i)the foaming temperature and (ii) the reaction start temperature of theheat cross-linking agent, and (b) heating the extruded mixture to atemperature higher than the activation temperatures of both thecross-linking agent and the heat foaming agent to cause foaming andcross-linking.
 15. The method of claim 14, wherein said heating stepcomprises heating the mixture to a temperature higher than theactivation temperatures of both the cross-linking agent and the heatfoaming agent to cause foaming and cross-linking at substantially thesame time.
 16. The method of claim 14, wherein said heating stepcomprises: heating to a temperature above the foaming temperature of theheat foaming agent but lower than the reaction start temperature of theheat cross-linking agent; and then heating to a temperature above thereaction start temperature of the heat cross-linking agent.
 17. Themethod of claim 14, further comprising: co-extruding or laminating anadditional layer to the pressure sensitive adhesive.
 18. The method ofclaim 14, wherein said mixture further comprises a non-cross-linkablerubber, elastomer, or thermoplastic polymer.
 19. The method of claim 6,wherein the mixture further comprises a radiation-impermeable componentselected from a pigment, a colorant, a metal filler, an inorganicfiller, or a combination thereof.